The present invention relates to printers of a small size, e.g., to miniature printers. More particularly, the invention relates to such printers in which a mechanism is employed for selecting print characters as a carriage is moved along a print line and then is returned to its home position.
Miniature printers are often used in desktop calculators, electronic cash registers, and the like, to move a print wheel to a desired print position in front of a piece of paper which is to be printed on, to rotate the wheel to select a desired print character, and then to print the character. In general, printers of this kind have a rotating mechanism for selecting print characters which is independent of the mechanism that is used for moving the print wheel along the print line. The structure of the printer is therefore complicated and is difficult, if not impossible, to manufacture in the small size which has been demanded.
An apparatus is disclosed in the U.S. Pat. No. 4,437,776 which attempts to solve this problem by using a drive in which a worm meshes with a worm gear that is made integral with the print wheel. A rack is moved in and out of engagement with the worm gear so that movement of the print wheel in rotation and in translation are accomplished in one mechanism. Another apparatus which addresses the problem is disclosed in Japanese Laid-Open Patent No. 107379/1983, where a worm gear which is integral with a print wheel meshes with two worms which rotate at different angular velocities, simultaneously rotating the wheel and moving it along the print line. The apparatuses of the foregoing use greatly simplified mechanisms in an effort to solve the aforementioned problem. However, in the apparatus of the U.S. patent, when the rack comes into and out of engagement with the worm gear, their addendums abut against each other so that precise operation is not feasible. In addition, a large amount of electric power is consumed for moving the print wheel along the print line because the mechanism which returns the print wheel to its original state uses a tension spring against which the drive motor must work. In the apparatus of the Japanese laid-open patent the mechanism moves the print wheel along the print line by using the difference between two angular worm drive velocities, and a long time is necessarily consumed in returning the print wheel to its original angular position. Also the drive must be reversed to return the print wheel to its original angular position. Hence, the device is not well-suited for high-speed printing. Conventional miniature printers also undesirably require additional power for shifting the print wheel between different levels of print type.